1. Field of the Invention
The invention relates to a knee airbag device mounted on the steering column of a vehicle.
2. Description of the Related Art
In a knee protecting airbag device as disclosed in, for example, Japanese Patent Application Publication No. 2008-110706 (JP-A-2008-110706), a case that houses an airbag is integrally formed with a column cover in which a flexible sheet material is embedded. The case includes a flexible portion consisting of the sheet material and a mounting portion mounted to the steering column. The flexible portion is located, in a longitudinal direction of the vehicle, in a front portion of the case that interferes with a stationary member of the steering column when an impact force acts on the steering column and a movable member of the steering column moves forward along the axial direction. The flexible portion is also configured to extend backward from the column cover in the longitudinal direction of the vehicle. The mounting portion is formed with higher rigidity than that of the flexible portion and is located in the rear portion of the flexible portion in the longitudinal direction of the vehicle. With this arrangement, the airbag device does not hamper movement of the movable member when the steering column absorbs the impact force, and is also arranged to receive reaction force when the airbag is deployed.
In the conventional airbag device as described above, however, the case that houses the airbag consists principally of the flexible seat material, and it is therefore difficult to maintain the shape of the folded knee airbag. Also, the knee protecting airbag device needs to be designed under many restrictions so that the knee airbag do not interfere with the steering column when a tilting operation or telescopic operation is performed on the steering column.